1. Field of the Invention
The present invention relates to a CMOS image sensor. More particularly, the present invention relates to a CMOS image sensor and a method of manufacturing the same wherein the wiring for electrically connecting the floating diffusion region and drive transistor of the image sensor is simplified.
2. Discussion of the Related Art
An image sensor is a device which is used for converting an optical image into an electrical signal. Typically, the image sensors currently used in the art are either complementary metal-oxide-silicon (CMOS) image sensors or charge coupled device (CCD) image sensors.
The CCD image sensor has excellent photo sensitivity and noise characteristics, but is difficult to incorporate in highly integrated devices and has a relatively high power consumption rate compared to the CMOS image sensor. The CMOS image sensor, on the other hand, has simpler processes and a lower power consumption rate, making it more suitable in highly integrated devices.
Recently, a technique for manufacturing a semiconductor device has been developed, wherein the CMOS image sensor has improved characteristics. Because of these developments, there has been a lot of interest in CMOS image sensors.
Generally, the CMOS image sensor comprises photodiodes capable of receiving light and transistors capable of controlling image signals input from the photodiodes. The CMOS image sensor is either a three T-type or a four T-type, depending on the number of transistors. Here, the three T-type sensor is includes one photo diode and three transistors, and the four T-type has one photo diode and four transistors.
A four T-type CMOS image sensor of the related art is illustrated in FIG. 1, and comprises a photo diode region (PD) formed at in an active region 1, at the portion of the active region 1 having the widest area. The image sensor also includes a transfer transistor (Tx), a reset transistor (Rx) and a drive transistor (Dx) formed so as to overlap with the active region 1 in a region other than the photo diode region (PD).
FIG. 2 illustrates the layers of the CMOS image sensor. The CMOS sensor comprises a P type epitaxial layer 4 is formed on a P++ type semiconductor substrate 2. Additionally the sensor includes a device isolating layer 6, a gate electrode 10, a gate oxide film 12 and a gate spacer 14, an n− type diffusion region 16, an n+ type diffusion region 18, an inter-layer dielectric layer 26, first and second contact holes 20 and 30, first and second contact plugs 22 and 32, and a metal wiring 24.
A device isolating layer 6 is formed in the device isolating region of the semiconductor substrate 2 at a location where the photo diode region (PD), the active region 1, and the device isolating region are located.
The gate electrode 10 is formed on a gate isolating layer 8 formed on the expitaxial layer 4 so as to form the transfer transistor (Tx) and the drive transistor (Dx). The gate oxide film 12 and the gate spacer 14 are formed on both walls of the gate electrode 10.
A n− type diffusion region 16 is formed in the epitaxial layer 4 of the photo diode region (PD). While a n+ type diffusion region 18 is formed at the active region 1 between the respective transistors (Tx, Rx, Dx).
An inter-layer dielectric layer 26 is formed on the epitaxial layer 4 in order to cover the gate electrode 10 including the gate spacer 14, and a first contact hole 20 is formed so as to penetrate through the inter-layer dielectric layer 26 in order to expose a floating diffusion region (FD). A second contact hole is formed so as to penetrate through the inter-layer dielectric layer 26 in order to expose the gate electrode 10 of the driver transistor (Dx). First and second contact plugs 22 and 32 are formed in the first and second contact holes 20 and 30 of the inter-layer dielectric layer 26, and metal wiring 24 is formed on the first and second contact plugs 22 and 32 in order to electrically connect the floating diffusion region (FD) and the gate electrode 10 of the driver transistor (Dx).
The photo diode (PD) senses incident light to generate charges in the sensor depending on the quantity of light. The transfer transistor (Tx) transfers the charges generated from the photo diode (PD) to the floating diffusion region (FD). Before the charges are transferred, the floating diffusion region (FD) moves electrons from the photo diode (PD) to the reset transistor (Rx) in order to turn the reset transistor on. Then, the floating diffusion region (FD) is set at a predetermined low-charge level. The reset transistor (Rx) discharges the charges stored in the floating diffusion region (FD) in order to detect a signal. The drive transistor (Dx) converts the charges into a voltage signal. Typically, metal wiring is used in order to connect the floating diffusion region (FD) and the drive transistor (Dx).
In comparison to the four T-type CMOS image sensor, a three T-type CMOS image sensor known in the related art is shown in FIG. 3. The tree T-type CMOS image sensor comprises a photo diode region (PD) formed at the widest portion of an active region, a reset transistor (Rx), and a drive transistor (Dx) formed to be overlapped with the active region in an area other than the photo diode region (PD). In the three T-type CMOS image sensors, metal wiring is used to electrically connect the photo diode (PD) and the drive transistor (Dx).
The metal wiring 40 is electrically connected to the various components by a first contact plug and a second contact plug (not shown). The first contact plug is formed in a first contact hole 42 wherein the gate electrode of the drive transistor (Dx) is exposed, and the second contact plug is formed in the second contact hole 44 where the photo diode (PD) is exposed.
Using these configurations, the three T-type and four T-type image sensors convert the charges generated from the photo diode region (PD) into a voltage signal at the drive transistor (Dx). Here, the metal wirings 24 and 40 are used in order to electrically connect the photo diode (PD) or the floating diffusion region (FD) and the drive transistor (Dx).
One difficulty in producing the CMOS image sensor of the related art is that manufacturing the metal wirings is complicated. Additionally, there is insufficient space for forming additional metal wirings for connecting additional components, such as a peripheral transistor without increasing the number of layers in the CMOS sensor.